The present invention relates to a transmission arrangement for digital signals operating with coded scanning samples and whose transfer function can be changed without interrupting operation and with low interference starting from an initial transfer function to a final transfer function, the arrangement including switching means for the transmission coefficients to be associated with each transmission member of the transmission arrangement for switching from a set of starting coefficients to a set of final coefficients. Such a transmission arrangement can generally be used for digital filtering, as well as for level control, with filter parameters, i.e. a set of coefficients for a transmission member, which are switched during operation. Voice, music and also inaudible signals can be used as digital signals.
The simplest method to change the transfer function of a transmission arrangement for digital signals during operation is to exchange the entire set of coefficients between two sampling times. However, such an abrupt, instantaneous switching of the set of coefficients, if no special measures are taken for interference suppression, results in interference in the output signal of the transmission arrangement, particularly--for example in connection with audio signals--interference which is annoying to the ear. For example, clicking noises result which, depending on the configuration and arbitrary selection of the moment of switching, have a more or less annoying effect.
It is conceivable to reduce such interference by including interference suppression members in the path of the digital signals. However, such solutions have not been found to be fully satisfactory because they influence the useful signal in an undesirable manner.
However, in Federal Republic of Germany Patent Application P 3,333,274.6, published April 18th, 1985, it was proposed to provide a second branch including a second digital transmission member in parallel with the transmission member whose set of coefficients is to be switchable. If a change in the transfer function of the first transmission member is desirable, instead of changing the coefficients of the first transmission member, the coefficients of the second transmission member are switched to the desired values. The output value of the second transmission member, however, is transferred by switching means to the output of the transmission arrangement only if a discriminator, which is connected to be effective with a delay equivalent to the transient period of the second transmission member, causes the switching of the switching means after completing a comparison of the output values of the two transmission members.
However, this transmission arrangement has a number of drawbacks, the most serious one being the fact that the expenditures for hardware or computations must be doubled to permit switching of coefficients.
In a further Federal Republic of Germany Application, P 3,333,275.4, published April 18th, 1985, it was proposed to associate the transmission members, by way of switching members, with intermediate coefficients (formed by interpolation values between a set of starting coefficients and a set of final coefficients) which are to assure that the transfer function of the transmission member changes in small increments from a starting transfer function to a final transfer function.
This transmission arrangement also still has some drawbacks.
1. To realize a noticeable improvement compared to the abrupt switching, a relatively large number of intermediate steps are necessary in some cases.
2. During these intermediate steps, the transmission coefficients are linearly interpolated independently of one another between their respective starting and final values. Under certain circumstances, the transfer functions associated with these intermediate coefficients may deviate considerably more from the initial or final transfer functions than from one another. This may cause the development of considerable interference signals during the switching process.
3. Special interpolation means are necessary to generate the intermediate coefficients.